U.S. patent number 5,870,243 [Application Number 08/824,522] was granted by the patent office on 1999-02-09 for servo frame defect mapping in a hard disc drive.
This patent grant is currently assigned to Seagate Technology, Inc.. Invention is credited to Daniel Eugene Hobson, Anish A. Ukani.
United States Patent |
5,870,243 |
Ukani , et al. |
February 9, 1999 |
Servo frame defect mapping in a hard disc drive
Abstract
Apparatus and method for detecting defective servo frames in a
disc drive, the defective servo frames including anomalous position
field patterns which generate burst signals of insufficient
relative magnitude to facilitate proper servo control. Burst
signals are generated from the position field patterns of a
selected servo frame and a combination term is determined as a
selected combination of the magnitudes of the burst signals. The
combination term is compared to a predetermined threshold and the
servo frame is identified as being defective when the combination
term falls below the predetermined threshold. The combination term
is preferably generated from the magnitudes of burst signals
generated from three burst patterns: a first burst pattern
extending from a first track boundary to a second, adjacent track
boundary, a second burst pattern extending from a track centerline
to the first track boundary and a third burst pattern extending
from the track centerline to the second track boundary.
Inventors: |
Ukani; Anish A. (Oklahoma City,
OK), Hobson; Daniel Eugene (Yukon, OK) |
Assignee: |
Seagate Technology, Inc.
(Scotts Valley, CA)
|
Family
ID: |
26698920 |
Appl.
No.: |
08/824,522 |
Filed: |
March 26, 1997 |
Current U.S.
Class: |
360/77.08;
360/75; 360/53; G9B/5.19 |
Current CPC
Class: |
G11B
5/5534 (20130101) |
Current International
Class: |
G11B
5/55 (20060101); G11B 005/596 () |
Field of
Search: |
;360/75,53,77.08,77.05,77.02,77.07 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hindi; Nabil
Attorney, Agent or Firm: Crowe & Dunlevy
Claims
What is claimed is:
1. In a disc drive of the type having a disc for the storage and
retrieval of data by a read/write head adjacent the disc, the disc
having prerecorded servo information defining a plurality of
nominally concentric tracks on the disc, the disc drive further
having a servo loop for using the servo information to control the
position of the head with respect to the tracks on the disc, the
servo information comprising a plurality of servo frames, each
servo frame comprising a position field including a first pattern
extending from a first track boundary to a second track boundary, a
second pattern extending from a track centerline halfway between
the first and second track boundaries to the first track boundary
and a third pattern extending from the track centerline to the
second track boundary, a method for detecting a defective servo
frame comprising steps of:
generating burst signals from the first, second and third patterns
of a selected servo frame associated with a selected track;
determining a combination term from a selected combination of the
burst signals;
comparing the combination term to a predetermined threshold;
and
identifying the servo frame as defective when the combination term
does not exceed the predetermined threshold.
2. The method of claim 1, wherein the burst signals comprise:
a first burst signal generated by the first pattern;
a second burst signal generated by the second pattern; and
a third burst signal generated by the third pattern; and
wherein the combination term is generated as the difference between
the first burst signal and the average of the second and third
burst signals.
3. The method of claim 1, wherein the servo information is
organized in a quadrature servo configuration.
4. In a disc drive of the type having a disc for the storage and
retrieval of data by a read/write head adjacent the disc, the disc
having prerecorded servo information defining a plurality of
nominally concentric tracks on the disc, the disc drive further
having a servo loop for using the servo information to control the
position of the head with respect to the tracks on the disc, the
servo information comprising a plurality of servo frames, each
servo frame comprising a position field including a first pattern
extending from a first track boundary to a second track boundary, a
second pattern extending from a track centerline halfway between
the first and second track boundaries to the first track boundary
and a third pattern extending from the track centerline to the
second track boundary, the improvement comprising:
defective servo frame means for identifying defective servo frames
of the drive, the defective servo frame means comprising:
combination term generation means, responsive to the head, for
generating a combination term as a selected combination of burst
signals generated from the first, second and third patterns of a
selected servo frame;
comparison means, responsive to the combination term means, for
comparing the combination term to a predetermined threshold;
and
defective servo frame identification means, responsive to the
comparison means, for identifying the servo frame as defective at
such time that the combination term does not exceed the
predetermined threshold.
5. The improvement of claim 4, wherein the burst signals
comprise:
a first burst signal generated by the first pattern;
a second burst signal generated by the second pattern; and
a third burst signal generated by the third pattern; and
wherein the combination term generation means generates the
combination term as the difference between the first burst signal
and the average of the second and third burst signals.
Description
RELATED APPLICATIONS
This application claims priority to Provisional Application Ser.
No. 60/024,839 filed Aug. 28, 1996.
FIELD OF THE INVENTION
This invention relates generally to the field of disc drive data
storage devices, and more particularly, but not by way of
limitation, to an apparatus and method for mapping defective servo
frames in a hard disc drive.
BACKGROUND OF THE INVENTION
Modern hard disc drives comprise one or more rigid discs that are
coated with a magnetizable medium and mounted on the hub of a
spindle motor for rotation at a constant high speed. Information is
stored on the discs in a plurality of concentric circular tracks by
an array of transducers ("heads") mounted to a radial actuator for
movement of the heads relative to the discs.
Typically, such radial actuators employ a voice coil motor to
position the heads with respect to the disc surfaces. The heads are
mounted via flexures at the ends of a plurality of arms which
project radially outward from an actuator body. The actuator body
pivots about a shaft mounted to the disc drive housing at a
position closely adjacent the outer extreme of the discs. The pivot
shaft is parallel with the axis of rotation of the spindle motor
and the discs, so that the heads move in a plane parallel with the
surfaces of the discs.
The actuator voice coil motor includes a coil mounted on the side
of the actuator body opposite the head arms so as to be immersed in
the magnetic field of a magnetic circuit comprising one or more
permanent magnets and magnetically permeable pole pieces. When
controlled DC current is passed through the coil, an
electromagnetic field is set up which interacts with the magnetic
field of the magnetic circuit to cause the coil to move in
accordance with the well-known Lorentz relationship. As the coil
moves, the actuator body pivots about the pivot shaft and the heads
move across the disc surfaces.
Control of the position of the heads is typically achieved with a
closed loop servo system such as disclosed in U.S. Pat. No.
5,262,907 entitled HARD DISC DRIVE WITH IMPROVED SERVO SYSTEM,
issued to Duffy et al., assigned to the assignee of the present
invention. A typical servo system utilizes servo information that
is written to the discs during the disc drive manufacturing process
to detect and control the position of the heads through the
generation of a position error signal (PES) which is indicative of
the position of the head with respect to a selected track. More
particularly, during track following in which the head is caused to
follow a selected track, the servo system generates the PES from
the received servo information and then uses the PES to generate a
correction signal which is provided to a power amplifier to control
the amount of current through the actuator coil, in order to adjust
the position of the head accordingly.
Typically, the PES is presented as a position dependent signal
having a magnitude indicative of the relative distance between the
head and the center of a track and a polarity indicative of the
direction of the head with respect to the track center. Thus, it is
common for the PES to have normalized values corresponding to a
range of, for example -1.0 to +1.0, as the head is swept across a
selected track and to have a value corresponding to a value of 0
when the head is positioned over the center of the track. As will
be recognized, modern servo systems typically generate the PES as a
sequence of digital samples which generally correspond to the above
analog range.
The PES is generated by the servo system by comparing the relative
signal strengths of burst signals generated from precisely located
magnetized servo fields in the servo information on the disc
surface. The servo fields are generally arranged in an "offset
checkerboard" pattern so that, through manipulation of the
magnitudes of the burst signals provided to the servo system as the
servo fields are read, the relative position of the head to a
particular track center can be determined and controlled. More
particularly, digital representations of the analog burst signals
are typically provided to a servo loop microprocessor (or digital
signal processor), which obtains a digital representation of the
value of the PES from a selected combination of the input digital
representations of the analog burst signals. The microprocessor
then compares the value of the PES to a desired value indicative of
the desired position of the head to the selected track and issues a
digital correction signal to the power amplifier, which in turn
provides an analog current to the actuator coil to adjust the
position of the actuator accordingly.
The servo information, including the servo fields, are written to
the discs during the manufacturing process using a highly precise
servo track writer. Although methodologies vary in the writing of
the servo information, typically the disc drive is mounted on the
servo track writer and the appropriate write signals are provided
to the heads of the disc drive to write the servo information while
the discs are rotated by the disc drive spindle motor. A mechanical
pusher arm is used to incrementally advance the heads over the
surfaces of the discs while a closed loop positional control system
ensures the heads are properly located relative to the discs.
Depending upon a particular configuration, each servo field is
typically written using a plurality of rotations of the disc, with
a portion of the servo field being written during each rotation of
the disc.
Although servo track writers have proven to be highly precise and
reliable (sufficient to support disc drive data storage areal
densities exceeding 1 Gbit/in.sup.2), errors have been found to
occasionally occur during servo track writing operations. For
example, it is common for a servo track writer to write a selected
servo field using a sinusoidal write signal of selected magnitude
and phase over a plurality of passes of the head so that a portion
of the field is written during each pass. If during one of the
passes the servo track writer erroneously uses a sinusoidal write
signal that is out of phase, the resulting servo field, though
precisely located on the disc, will produce a burst signal having a
reduced magnitude. Because the servo system relies upon the
relative magnitudes of the servo fields, such reduction in
magnitude can adversely affect the ability of the servo system to
discern the location of the head with respect to the track and
control the position of the head.
Moreover, localized anomalies in the media can prevent the
generation of burst signals having the proper relative magnitudes,
even when the servo fields have been otherwise properly formed
during the servo track write operation.
Accordingly, there is a need for an improved approach to detecting
the defective servo frames including anomalous position field
patterns which generate burst signals of insufficient relative
magnitude to facilitate proper servo control.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for detecting
defective servo frames in a disc drive, the defective servo frames
including anomalous position field patterns which generate burst
signals of insufficient relative magnitude to facilitate proper
servo control.
In accordance with the preferred embodiment, the disc drive enters
a defect mapping routine wherein burst signals are generated from
the position field patterns of a selected servo frame and a
combination term is determined as a selected combination of the
magnitudes of the burst signals. The combination term is compared
to a predetermined threshold and the servo frame is identified as
defective at such time that the combination term is less than the
predetermined threshold.
The combination term is preferably generated from the magnitudes of
burst signals generated from three burst patterns: a first burst
pattern extending from a first track boundary to a second, adjacent
track boundary, a second burst pattern extending from a track
centerline to the first track boundary and a third burst pattern
extending from the track centerline to the second track boundary.
More particularly, the combination term is preferably determined as
the difference between the first pattern burst signal and the
average of the second and third pattern burst signals. A one-half
track offset can also be applied to the servo loop so that
different combinations of the burst patterns are tested by the
defect mapping routine.
These and various other features as well as advantages which
characterize the present invention will be apparent from a reading
of the following detailed description and a review of the
associated drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a disc drive constructed in accordance with the preferred
embodiment of the present invention.
FIG. 2 provides a functional block diagram of the disc drive of
FIG. 1 operably connected to a host computer in which the disc
drive can be mounted.
FIG. 3 provides a functional block diagram of a servo control
circuit shown in FIG. 2.
FIG. 4 provides a representation of the general format of a servo
frame used by the servo control circuit of FIG. 3.
FIG. 5 shows A, B, C and D burst patterns of the servo frame of
FIG. 2.
FIG. 6 provides a graphical representation of the amplitudes of A,
B, C and D burst signals generated from the burst patterns of FIG.
3.
FIG. 7 shows an idealized representation of a linear position error
signal generated from the burst signals of FIG. 4.
FIG. 8 is a generalized flow chart for a servo defect mapping
routine, preferably stored in the form of programming in the servo
RAM of FIG. 1 and executed by the servo microprocessor of FIG. 1 in
accordance with the preferred embodiment of the present
invention.
DETAILED DESCRIPTION
Referring now to FIG. 1, shown therein is a disc drive 100
constructed in accordance with the preferred embodiment of the
present invention. The disc drive 100 includes a base deck 102 to
which various components of the disc drive 100 are mounted. A top
cover 104 (shown in partial cutaway fashion) cooperates with the
base deck 102 to form an internal, sealed environment for the disc
drive in a conventional manner.
A spindle motor (shown generally at 106) rotates one or more discs
108 at a constant high speed. Information is written to and read
from tracks (not designated) on the discs 108 through the use of an
actuator assembly 110, which rotates about a bearing shaft assembly
112 positioned adjacent the discs 108. The actuator assembly 110
includes a plurality of actuator arms 114 which extend towards the
discs 108, with one or more flexures 116 extending from the
actuator arms 114. Mounted at the distal end of each of the
flexures 116 is a head 118 which includes a slider assembly (not
separately designated) designed to fly in close proximity to the
corresponding surface of the associated disc 108.
At such time that the disc drive 100 is not in use, the heads 118
are moved over landing zones 120 near the inner diameter of the
discs 108 and the actuator assembly 110 is secured using a
conventional latch arrangement, such as designated at 122.
The radial position of the heads 118 is controlled through the use
of a voice coil motor (VCM) 124, which as will be recognized
typically includes a coil 126 attached to the actuator assembly 110
as well as one or more permanent magnets 128 which establish a
magnetic field in which the coil 126 is immersed. Thus, the
controlled application of current to the coil 126 causes magnetic
interaction between the permanent magnets 128 and the coil 126 so
that the coil 126 moves in accordance with the well known Lorentz
relationship. As the coil 126 moves, the actuator assembly 110
pivots about the bearing shaft assembly 112 and the heads 118 are
caused to move across the surfaces of the discs 108.
A flex assembly 130 is provided to provide the requisite electrical
connection paths for the actuator assembly 110 while allowing
pivotal movement of the actuator assembly 110 during operation. The
flex assembly includes a printed circuit board 132 to which head
wires (not shown) are connected, the head wires being routed along
the actuator arms 114 and the flexures 116 to the heads 118. The
printed circuit board 132 typically includes circuitry for
controlling the write currents applied to the heads 118 during a
write operation and for amplifying read signals generated by the
heads 118 during a read operation. The flex assembly terminates at
a flex bracket 134 for communication through the base deck 102 to a
disc drive printed circuit board (not shown) mounted to the bottom
side of the disc drive 100.
Referring now to FIG. 2, shown therein is a functional block
diagram of the disc drive 100 of FIG. 1, generally showing the main
functional circuits which are resident on the disc drive printed
circuit board and used to control the operation of the disc drive
100.
The disc drive 100 is shown to be operably connected to a host
computer 140 in which the disc drive 100 is mounted in a
conventional manner. Control communication paths are provided
between the host computer 140 and a disc drive microprocessor 142,
the microprocessor 142 generally providing top level communication
and control for the disc drive 100 in conjunction with programming
for the microprocessor 142 stored in microprocessor memory (MEM)
143. The MEM 143 can include RAM, ROM and other sources of resident
memory for the microprocessor 142.
Data is transferred between the host computer 140 and the disc
drive 100 by way of a disc drive interface 144, which typically
includes a buffer to facilitate high speed data transfer between
the host computer 140 and the disc drive 100. Data to be written to
the disc drive 100 are thus passed from the host computer to the
interface 144 and then to a read/write channel 146, which encodes
and serializes the data and provides the requisite write current
signals to the heads 118. To retrieve data that have been
previously stored by the disc drive 100, read signals are generated
by the heads 118 and provided to the read/write channel 146, which
performs decoding and error detection and correction operations and
outputs the retrieved data to the interface 144 for subsequent
transfer to the host computer 140. Such operation of the disc drive
100 is well known in the art and discussed, for example, in U.S.
Pat. No. 5,276,622 issued Jan. 4, 1994 to Shaver et al assigned to
the assignee of the present invention.
The discs 108 are rotated at a constant high speed by a spindle
control circuit 148, which typically electrically commutates the
spindle motor 106 (FIG. 1) through the use of back emf sensing.
Spindle control circuits such as represented at 148 are well known
and will therefore not be discussed further herein; additional
information concerning spindle control circuits is provided in U.S.
Pat. No. 5,631,999 issued May 20, 1997 to Dinsmore, assigned to the
assignee of the present invention.
As discussed above, the radial position of the heads 118 is
controlled through the application of current to the coil 126 of
the actuator assembly 110. Such control is provided by a servo
control circuit 150, a functional block diagram of which is
provided in FIG. 3.
Referring now to FIG. 3, the servo control circuit 150 includes a
preamp circuit 152, a servo data and decode circuit 154, a servo
processor 156 with associated servo RAM 158 and a VCM control
circuit 160, all of which cooperate in a manner to be discussed in
greater detail below to control the position of the head 118. For
reference, the preamp circuit 152 is typically located on the
printed circuit board 132 (FIG. 1) as it has been found to be
generally advantageous to locate the preamp circuit 152 in close
proximity to the heads 118.
It will be recognized that servo control generally includes two
main types of operation: seeking and track following. A seek
operation entails moving a selected head 118 from an initial track
to a destination track on the associated disc surface through the
initial acceleration and subsequent acceleration of the head 118
away from the initial track and towards the destination track. Once
the head 118 is settled on the destination track, the disc drive
enters a track following mode of operation wherein the head 118 is
caused to follow the destination track until the next seek
operation is to be performed. Such operations are well known in the
art and are discussed, for example, in the previously referenced
Duffy U.S. Pat. No. 5,262,907 as well as in U.S. Pat. No. 5,475,545
issued Dec. 12, 1995 to Hampshire et al., assigned to the assignee
of the present invention. In order to clearly set forth the
preferred embodiment of the present invention, however, the general
operation of the servo control circuit 150 during track following
will now briefly be discussed.
With continued reference to FIG. 3, analog burst signals are
provided by the head 118 at such time that servo information
associated with the track being followed passes under the head 118.
The burst signals are amplified by the preamp circuit 152 and
provided to the servo data decode circuit 154, which includes
analog-to-digital converter (ADC) circuitry that converts the
analog burst signals to digital form. The digitized signals are
then provided to the servo processor 156, which in the preferred
embodiment is a digital signal processor (DSP).
The servo processor 156 determines a position error signal from the
relative magnitudes of the digital representations of the burst
signals and, in accordance with commands received from the disc
drive microprocessor 142 (FIG. 2), determines the desired position
of the head 118 with respect to the track. It will be recognized
that, generally, the optimal position for the head 118 with respect
to the track being followed is over track center, but offsets (as a
percentage of the width of the track) can sometimes be
advantageously employed during, for example, error recovery
routines. In response to the desired relative position of the head
118, the servo processor 156 outputs a current command signal to
the VCM control circuit 160, which includes an actuator driver (not
separately designated) that applies current of a selected magnitude
and direction to the coil 126 in response to the current command
signal.
The servo information on the discs 108 is recorded during the
manufacturing of the disc drive 100 using a highly precise servo
track writer. The servo information serves to define the boundaries
of each of the tracks and is divided circumferentially into a
number of frames, the general format of which is shown in FIG. 6.
More particularly, FIG. 6 shows a frame 170 to comprise a plurality
of fields, including an AGC & Sync field 172, an index field
174, a track ID field 176 and a position field 180. Of particular
interest is the position field 180, but for purposes of clarity it
will be recognized that the AGC & Sync field 172 provides input
for the generation of timing signals used by the disc drive 100,
the index field 174 indicates radial position of the track and the
track ID field 176 provides the track address. Of course, other
fields may be used as desired and the format of the fields in a
servo frame will depend upon the construction of a particular disc
drive.
The position field 180 comprises four position burst fields
arranged in an offset, quadrature pattern for a plurality of
adjacent tracks, as shown in FIG. 5. More particularly, FIG. 5
shows the position field 180 to comprise burst patterns A, B, C and
D having selected geometries and magnetization vectors, defining a
plurality of track boundaries identified as 0-5. Thus, each track
comprises the area bounded by two adjacent track boundaries.
Additionally, the head 118 of FIG. 1 is represented in FIG. 3 as
being centered on the track bounded by track boundaries 0 and 1
(said track being identified at 182). The direction of rotation of
the discs 108 (and hence the position field 180) relative to the
head 118 is shown by arrow 184.
Both the A and B burst patterns are shown to extend from the center
of one track to the center of an immediately adjacent track, with
these patterns offset as shown. Additionally, the C and D burst
patterns extend from one track boundary to the next track boundary,
with these patterns also offset as shown. Thus, as the head 118
passes over the position field 180 on track 182, the head will pass
over portions of the A and B burst patterns (identified as 186 and
188, respectively) and then over C burst pattern 190. However, the
head 118 will not encounter D burst pattern 192, as this pattern is
on an adjacent track. For reference, tracks having C burst patterns
are referred to as "even tracks" and tracks with D burst patterns
are referred to as "odd tracks".
Generally, it will be recognized that when the head 118 is centered
at the mid-point of track 182, the amplitude of an A burst signal
induced in the head 118 by the A burst pattern 186 will be
nominally equal to the amplitude of a B burst signal induced in the
head by the B burst pattern 188. Moreover, the amplitude of a C
burst signal induced by the C burst pattern 190 will have a nominal
maximum value and the amplitude of a D burst signal from the D
burst pattern 192 will be nominally zero. Further, when the head
118 is positioned over the track boundary 1, the amplitudes of the
C and D burst signals from the patterns 190 and 192 will be equal
in magnitude, the B burst signal from the pattern 188 will have a
maximum value and the A burst from the pattern 186 will be zero.
Thus, as the head 118 is swept from one track boundary to the next,
the amplitudes of the A, B, C and D burst signals cycle between
zero and maximum values, as generally illustrated by FIG. 6.
FIG. 6 provides a graphical representation of the amplitudes of the
A, B, C and D burst signals as the head 118 is moved from track
boundary 0 to track boundary 4 in FIG. 5. More particularly, FIG. 6
plots each of the burst signals along a common horizontal axis
indicative of radial track position and an aligned vertical axis
indicative of the amplitude for each of the burst signals from a
value of zero to a maximum value. As in FIG. 5, the track 182 is
shown in FIG. 6 to comprise the interval between the values of 0
and 1 on the horizontal axis.
Referring to FIG. 7, shown therein is a graphical representation of
an idealized PES curve 194 generated from the burst signals of FIG.
6. The PES curve 194 has an amplitude that generally ranges in a
linear fashion from a minimum value of -1 to a maximum value of +1
as the head is positioned across a track from one track boundary to
the next. That is, the PES has a nominal value of zero when the
head 118 is positioned at the center of a selected track and the
PES increases and decreases, respectively, in a linear fashion as
the head is positioned toward the track boundaries. In this way,
the amplitude and polarity of the PES curve 194 readily indicate
the relative distance and direction of the position of the head 118
with respect to a selected track center and can thus be used to
generate the appropriate correction signal to move the head to the
center of the selected track. It will be understood that, in the
digital servo control circuit 150 of FIGS. 2 and 3, the PES
comprises a range of digital values across each track from one
track boundary to the next; however, it is conventional to express
the relative values of the PES in a normalized, analog fashion as
shown on the vertical axis of FIG. 7.
It will be apparent that positional control of the head 118 is
achieved through the discernment by the servo control circuit 150
of the relative magnitudes of the burst signals generated as the
head 118 passes over the position field 180. However, should one or
more of the patterns 186, 188, 190 and 192 have an improper
magnetization arising as a result of an error during the servo
track write process (or for other causes, such as a localized
anomaly in the disc media), the relative magnitudes of the burst
signals received by the servo control circuit 150 may not reflect
the actual position of the head 118.
Referring again to FIG. 6, it will be noted that when the head 118
is disposed over the center of a selected track, the A and B burst
signals will generally be equal in magnitude to each other within a
certain tolerance and the C or D burst signal (depending upon
whether the track is even or odd) will have a magnitude that is
significantly greater than that of either the A or B burst signal.
Particularly, with the quadrature pattern disclosed in the
preferred embodiment wherein all patterns have nominally the same
magnetization strength, the C or D burst signal magnitude will
nominally be about twice the average of the A and B burst signal
magnitudes; that is, C.apprxeq.(A+B) for even tracks and
D.apprxeq.(A+B) for odd tracks, or for a given track, (C+D) will
generally be equal to (A+B).
Turning now to FIG. 8, shown therein is a generalized a servo
defect mapping routine, preferably stored in the form of
programming in the servo RAM 158 of FIG. 3 and executed by the
servo processor 156 of FIG. 3 in accordance with the preferred
embodiment of the present invention. Particularly, the routine of
FIG. 8 is used to determine whether the patterns (such as the
patterns 186, 188, 190 and 192 of FIG. 5) produce burst signals of
sufficient magnitude to facilitate proper servo loop operation. It
is contemplated that the routine will preferably be performed
during manufacturing of the disc drive 100, although it can also be
performed during field operation as desired. As described below,
servo frames which fail the routine are identified as defective and
marked accordingly.
The routine of FIG. 8 is shown to begin at block 202 wherein the
first servo frame (such as the frame 70 of FIG. 4) is selected. As
the routine is contemplated as being performed on all tracks of the
disc drive 100, the operation of block 202 includes the selection
of the first head (such as the head 118) and the performance of a
seek operation whereby the servo control circuit 150 moves the head
118 over a first track (such as at the inner diameter of disc 118
of FIG. 3). Once over the first track, the servo control circuit
150 enters a track following mode of operation so that the head 118
is nominally disposed over the center of the first track and
follows the first track as the disc 108 rotates. For reference, in
an embedded servo system such as disclosed herein, each track will
have from about 30 to 90 servo frames with user data disposed
therebetween.
Once the first frame is selected, the routine of FIG. 8 continues
to block 204 wherein, at such time that the first frame passes
under the head 118, the magnitudes of the burst signals generated
by the associated A, B, C and D patterns are measured. Once
measured, a signal combination term T.sub.c is determined as
follows:
for even tracks, and
for odd tracks. The combination term T.sub.c is derived from the
general observation that (A+B)-(C+D) will be close to zero for a
good set of patterns. Although the combination term could readily
be determined in an alternative manner to that presented in
equations (1) and (2), such as T.sub.c
=.vertline.(A+B)-(C+D).vertline., the methodology of equations (1)
and (2) has been found to be preferable due to intermittent signal
values that have been received in a particular application when
both C and D patterns are used in the same calculation.
Continuing with the routine of FIG. 8, the combination term is then
compared to a predetermined threshold, as indicated by decision
block 206. The threshold is selected based upon the requirements of
a particular application, but generally represents the minimum
acceptable value for the combination term which will allow proper
operation by the servo control circuit 150. In the preferred
embodiment, a population of combination terms from good servo
frames are measured and the threshold is selected from this
population; for example, the threshold can be advantageously
selected as a value generally one-half of the minimum combination
term in the population.
Should the combination term exceed the threshold, the frame is
determined to be good and the routine continues to block 208,
wherein the next frame to be tested is selected. Should the
combination term not exceed the threshold, however, as shown in
FIG. 8 the frame is determined to be defective and marked
accordingly, as shown by block 210. The routine is thereafter
repeated until all of the frames have been tested, as indicated by
decision block 212, after which the routine ends at block 214.
The defective servo frame can be marked in a variety of ways known
in the art, including being designated in a defect map used by the
disc drive 100 during subsequent address allocations for data to be
stored on the discs 108. The servo control circuit 150 can then
ignore servo information provided by the defective frame when the
associated head is caused to follow the track containing the
defective frame and instead use estimates of head position,
velocity and acceleration determined from the servo frame
immediately preceding the defective servo frame. Additionally, the
sector or sectors associated with each defective servo frame can be
deallocated from future use by the disc drive 10.
From a review of equations (1) and (2) above it will be recognized
that the routine of FIG. 8 is particularly well suited to detect
defective servo frames having anomalous C or D patterns, but in
some cases servo frames having anomalous A or B patterns might
still provide a compensation term above the predetermined threshold
and therefore may not be detected as defective frames. Accordingly,
in such cases where it is important to ensure that the A and B
patterns are also correct, the routine of FIG. 8 can be repeated
with the additional use of a 1/2 track offset so that the servo
control circuit 150 servos off of the C and D patterns and
alternatingly tests the A and B patterns, using the following
equations for the compensation term:
Finally, although for purposes of disclosure a quadrature servo
configuration has been disclosed herein, it will be recognized that
the present invention can readily be adapted for use with other
servo system configurations.
It will be clear that the present invention is well adapted to
carry out the objects and attain the ends and advantages mentioned
as well as those inherent therein. While a presently preferred
embodiment has been described for purposes of this disclosure,
numerous changes may be made which will readily suggest themselves
to those skilled in the art and which are encompassed in the spirit
of the invention disclosed and as defined in the appended
claims.
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